CAES energy storage technology is based on the storage of compressed atmospheric air. When there is a surplus of energy in the electric power grid, the compressor is activated, compressing atmospheric air to a predetermined pressure, consuming the excess power from the mains. The resulting compressed air is stored in a natural cave, abandoned mine or salt dome cavern (a tank cannot be used due to the huge dimensions it should have and the high pressures it would have to withstand, if large amounts of energy want to be stored).
In order to keep the energy stored until energy demand rises again in the power grid, it is enough to maintain the cave closed by a shut-off valve. When the power grid finally requires more energy, the valve opens and pressurized air is vented out and used to drive a turbine and generate electricity.
Building plants with this technology can be much more economically feasible than building pumped storage hydro plants, the most widespread large scale energy storage system that exists today. But the main disadvantage of CAES technology is the low efficiencies that can be achieved, resulting in a very low feasibility of the operation of the plants. In fact, although it is a technology developed over thirty years ago, there are only two CAES plants operating in the world today.
In addition, the enormous demand on cavern volume requirements and pressures to withstand reduces dramatically the range of possible locations for this type of plants, being restricted, as explained, to places where there are natural caves, abandoned mines or salt domes.
It is difficult to find available natural caverns or abandoned mines, so that both CAES plants operating today were built using salt domes. This involves a number of additional problems such as the inability to thermally insulate the caverns (the excavation is performed by dissolving salts through a borehole about 600 m deep), or high levels of suspended particles in the air when it comes out of the cavern.